The present invention relates to a system for measuring the angle of arrival of an RF signal, using only two widely spaced antennas.
As shown in FIG. 1, the angle of arrival .theta. of an RF signal is given by the relation ##EQU1## where D is the distance or spacing between antennas A and B, d is the remaining distance to antenna A from the RF wavefront upon reaching antenna B, and where the RF source is sufficiently distant so that parallel RF signal paths extend to antennas A and B.
In one known system for determining differential distance d, the time difference of arrival is measured, which enables distance d to be calculated from the known frequency. The RF signal has pulse envelopes of RF carrier cycles. The antennas are widely spaced by many wavelengths and the time difference of arrival of the RF pulse envelopes is measured. This system requires extremely fast rise time pulse envelopes for good accuracy and/or high speed sampling and mathematical correlation of a digitized waveform to determine time delay which matches up detected pulse envelopes from two antennas. Even with these features, accuracy is severely limited.
In another known system, the RF carrier phase difference within the envelope is measured, from which d can be calculated from the known wavelength. This system, however, yields ambiguous results, wherein phase difference will be multiples of 180.degree., unless antennas are spaced less than a wavelength apart, or multiple antenna pairs are used to resolve the ambiguity.
The present invention addresses and solves the conflicting problems of the above systems. The invention provides a highly accurate system with only a single pair of widely spaced antennas, separated by more than a wavelength. Furthermore, there is no need to resort to multiple antenna pairs to resolve ambiguity.